Do not attempt to restart systemd services which do not exist

The first time float is run the services associated with credentials
won't have been installed yet, which causes an error.

roles/float-util-credentials/tasks/main.yml

@@ -6,6 +6,14 @@
     name: x509ca
     state: present
 
+# Get the list of currently known systemd service units. Unfortunately
+# the systemd version in Debian Buster does not yet support --output
+# to set a reasonable format, so we have to use 'awk'.
+# This will be used to restart services only if they actually exist.
+- name: List systemd service units
+  shell: "systemctl list-units --no-legend --no-pager --full --type service | awk '{print $1}'"
+  register: all_systemd_units
+
 # Get the credential names from the list of certs.
 - set_fact:
     credentials_names: "{{ credentials | map(attribute='credentials') | map(attribute='name') | unique | list }}"
@@ -95,7 +103,7 @@
 
     # This should use the systemd module but it doesn't take lists of services.
     - name: "Restart associated services"
-      shell: "systemctl restart {{ services[item.0.service].systemd_services | join(' ') }}"
+      shell: "systemctl restart {{ all_systemd_units.stdout_lines | intersect(services[item.0.service].systemd_services) | join(' ') }}"
       when: "item.1.changed and item.0.service != 'LOCAL' and services[item.0.service].systemd_services"
       loop: "{{ credentials | zip(x509_sign.results) | list }}"